A SMPS is used in many applications requiring a low cost single stage solution to provide isolated and regulated DC output power with good power factor correction and moderate line current distortion. The control circuit used for the SMPS is one developed for the SMPS boost topology that requires a second stage to provide isolation and output voltage scaling. These controllers are readily available and are low in cost, making them attractive for a low cost SMPS that may be used for consumer and commercial applications, e.g., voltage step-down supplies for lighting applications such as, for example but not limited to, light emitting diode (LED) luminaires, etc.
A SMPS integrated circuit controller may be, for example but is not limited to, a transition-mode power factor corrected (PFC) controller, such as device number L6562, manufactured by STMicroelectronics. The datasheet for the L6562 controller is available from STMicroelectronics at www.st.com, and is incorporated by reference herein for all purposes. The SMPS controller typically has voltage input for biasing the controller, Vcc, that has a turn-on threshold voltage, VTon, and a turn off voltage threshold, VToff, where VTon>VToff. This hysteresis between VTon and VToff allows for the Vcc capacitor to have enough stored energy such that the controller may start the switching of the SMPS, allowing for the energy processed by the SMPS to regenerate the Vcc bias capacitor sustaining operation into steady state operation. For the L6562, VTon=12.5 volts and VToff=10 volts. There are two paths for the Vcc power supply internal to the controller integrated circuit to derive source energy, the start-up path and the regenerative path. The start-up path is a circuit that will charge up a Vcc capacitor for operation of the controller above VTon such that the controller will start switching the power supply allowing it to process energy. As the controller starts switching the power converter, the controller consumes energy off the Vcc terminal resulting in the Vcc voltage to start falling. The regenerative path must then take some of this energy processed by the power converter and provide it to the Vcc capacitor such that the voltage does not fall below VToff. At this point the Vcc is stable in the power converter transitions into steady state operation.
When power is first applied to the SMPS, the power supply Vcc capacitor is trickle charged to a voltage sufficient to start operation of an SMPS integrated circuit controller. With the trickle charge method, a simple resistor is added from the power source to the Vcc storage capacitor such that when power is supplied to the controller circuits, current flows directly from the power source to the Vcc storage capacitor. This results in the Vcc voltage increasing until it reaches VTon, at which point the power converter starts to switch and process energy. Since the controller integrated circuit is now drawing energy, the Vcc voltage on the storage capacitor starts to fall as the controller integrated circuit drains current from the Vcc storage capacitor. As long as the regenerative path replenishes the energy on the Vcc storage capacitor before the Vcc voltage falls below VToff, the SMPS continues to run in an intended fashion.
If the power source voltage is constant, the SMPS designer may size the trickle charge resistor such that the power converter starts in a reasonable amount of time. The lower the resistance value the faster the circuit will start up by allowing more current to trickle charge to the Vcc capacitor. However, by using a lower the resistance value more power is dissipated during steady state operation due to E2/R (power) losses. Therefore, there is an optimal value that equally balances start-up time and power losses.
Also, when the power supply is a LED driver that is dimmable by a line voltage dimmer, the power supply can suffer increased start-up times that are unreasonably delayed when the light is switched on with the dimmer in a reduced light output setting. This is because the dimmer dims the light by reducing the RMS voltage input to the LED driver SMPS. With the voltage available reduced to the trickle charge path, the amount of current available to charge the Vcc capacitor is reduced resulting in longer start-up times. With the dimmer at its minimum setting the start-up time can be as long as five minutes. This is clearly unacceptable to a end user wishing to turn their lights on.